Portable communication terminals such as PHS (Personal Handy-phone System), PDA (Personal Digital Assistant) and the like are popularized as communication means. Such portable communication terminals (refereed to as cellular phones below) are connected to a communication network such as a public telephone network by way of the nearest one of a plurality of wireless base stations, each having a relatively narrow service area for enabling a terminal to be connected to another communication device such as a cellular hone, a stationary telephone, a PC (personal computer), etc. Such cellular phones are increasingly multi-function and high performance.
On the other hand, PCs are also popularized and undergo multi-function and high performance at a fast speed. For example, PCs having built-in wireless LAN have been developed. Due to cheaper operation cost, faster communication rate and convenience of wireless LAN, PCs having built-in wireless LAN are also rapidly popularized as data terminals or communication terminals.
Conventional cellular phone technologies as described hereinabove are disclosed in various technical documents. A communication system, a communication method and a communication control device for enabling to communicate between communication terminals by way of communication control devices that are interconnected through a network are disclosed (see, for example, Japanese patent publication no. 2003-69626). And a protocol conversion communication method that temporarily allocates a MAC (Media Access Control) address to a PPP (Point-to-point Protocol) session and releases such MAC address at the release of the session and a relay unit having such conversion function are disclosed (see, for example, Japanese patent publication no. 2003-234795). Moreover, a user confidentiality guarantee method that generates temporary addresses from a MAC address for selecting a particular one of the temporary addresses and a wireless LAN system therefor are disclosed (see, for example, Japanese patent publication no. 2004-40806).
FIG. 5 is a block diagram for illustrating the basic construction of the conventional cellular phone having built-in wireless LAN. In FIG. 5, the cellular phone having built-in wireless LAN 50 comprises a cellular phone function section 51, a wireless LAN connection (or link) control section 52, a wireless LAN transmitter/receiver section 53 and a cellular phone network transmitter/receiver section 54.
Now, describing the construction of these constituent elements more in detail, the cellular phone function section 51 includes such functions as voice communication, TV phone, E-mail, WEB browser and the like. The wireless LAN connection control section 52 is connected to the wireless LAN transmitter/receiver section 53. The wireless LAN transmitter/receiver section 53 and the cellular phone network transmitter/receiver section 54 are also connected to the cellular phone function section 51. The wireless LAN transmitter/receiver section 53 is connected to the wireless LAN base station 61a or 61b and then to a communicating terminal, server or the like by way of internet 62. On the other hand, the cellular phone network transmitter/receiver section 54 is connected to a communicating terminal, server or the like by way of the nearby cellular phone base station 63 and the cellular phone network 64.
The cellular phone 50 having the construction as described hereinabove is believed to be easily connected to the wireless LAN base station 61 by its user unlike a notebook PC. Even if it is not connected, a search for nearby wireless base stations 61 is conducted periodically in order to confirm if the wireless LAN is available.
Now, FIG. 6 is a sequence diagram to illustrate communication steps (or protocol) between the conventional cellular phone having built-in wireless LAN 50 and the wireless LAN base station 61 or the like as shown in FIG. 5. Firstly, the wireless LAN connection control section 52 of the cellular phone 50 requests search/connection to the wireless LAN transmitter/receiver section 53 (step A1). In response thereto, the wireless LAN transmitter/receiver section 53 sends a search request/data transmission to the nearby wireless LAN base station 61a (step A2). The wireless LAN base station 61a, then, returns search response/data transmission to the wireless LAN transmitter/receiver section 53 (step A3). Upon detecting disconnection from the wireless LAN base station 61a, the wireless LAN transmitter/receiver section 53 sends the disconnection detection to the wireless LAN connection control section 52 (step A4).
Then, assuming that the cellular phone 50 moves, the wireless LAN connection control section 52 of the cellular phone 50 sends search/connection to the wireless transmitter/receiver section 53 (step A5). Then, the wireless LAN transmitter/receiver section 53 sends search request/data transmission to another wireless LAN base station 61b (step A6). The wireless LAN base station 61b sends search response/data transmission to the wireless LAN transmitter/receiver section 53 of the cellular phone 50 (step A7) and the wireless LAN transmitter/receiver section 53 sends disconnection detection to the wireless LAN connection control section 52 (step A8). If the cellular phone 50 moves again, the search/connection (step A9), the search request/data transmission (step A10) and the search response/data transmission (step A11) will be performed in the similar manner as described hereinabove.
As illustrated in FIG. 6, the wireless LAN transmitter/receiver section 53 also performs search request/data transmission to a false base station or an observer 60. This means that the false base station or the observer 60 is able to acquire sender's addresses by performing pinpoint observation or relativity tracking.
Since wireless LAN devices are less expensive, communication monitors are also available at a low cost. Although a wireless LAN communicates by using its proper MAC address, such MAC address cannot be encrypted because it is contained in the header portion of the data. Even if it is not connected to the wireless LAN base station 61, a control packet is transmitted only for the purpose of conducting a search if there is any wireless LAN base station in the surroundings. Such packet enables one to observe the proper MAC address of the cellular phone 50 by using a communication monitor. Since wireless LAN devices are less expensive, it is also easy to operate a wireless LAN base station having no restriction in connection. If connection is made to a wireless LAN base station (for example, the false base station 60), the owner of the wireless LAN base station is able to easily identify the MAC address. This means that privacy (private information) may be acquired or leaked.
As described hereinabove, it is possible to monitor the place of the user of the cellular phone 50 at a certain time by monitoring the MAC address of the wireless LAN built in his/her cellular phone, thereby making it difficult to protect the user's privacy. As a solution to this problem, it is possible to randomly change the MAC address so that the user of the cellular phone cannot be identified. However, there is a possibility that the MAC address collides with one of another user in the same or near area, thereby causing impaired communication or unintentional communication hijacking. Although it is also possible to make arbitration of MAC addresses among nearby users in advance, such approach is useless if users having the same MAC address move close to each other while in communication.
Moreover, in public wireless LAN, a certification method that does not depend on MAC address has been proposed and is in practical use. However, such method poses another problem to lose compatibility with the existing wireless LAN system that uses terminals' MAC addresses for restriction.